Chapter Two

Can matter be divided endlessly into smaller and smaller pieces, or is there a point at which it can not be divided any further?

Democritus (460-370 B.C.) thought the latter.

Dalton’s atomic theory of matter:

• Each element is composed of extremely small particles called atoms.
• All atoms of a given element are identical; the atoms of different elements are different and have different properties (including different masses).
• Atoms of an element are not changed into different types of atoms by chemical reactions.
• Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.
  

Dalton’s theory explains several simple laws of chemical combination that were known in his time.

• The law of constant composition (In a given compound the relative numbers and kinds of atoms are constant).
• The law of conservation of mass (the total mass of materials present after a chemical reaction is the same as the total mass before the reaction).
• Dalton used his theory to deduce the law of multiple proportions.
  

By 1850, scientists had begun to accumulate data indicating that the atom is composed of even smaller particles called subatomic particles.

Experiments using cathode ray tubes lead early scientists came to discover-that the structure of the atom is somehow related to electrical charge.

There are only two kinds of electrical charges, Positive (+) ones and negative (-) ones.
Like charges repel each other; unlike charges attract.

In the early 1900s J. J. Thomson reasoned that because electrons comprise only a very small fraction of the mass of the atom they were responsible for an equally small fraction of the atom’s size.
He proposed the “plum-pudding” model which was very short-lived.

The spontaneous emission of radiation is called radioactivity.
Ernest Rutherford revealed that radiation has three components:

• alpha (a),
• beta (b),
• gamma (g)
  

Alpha particles are helium nuclei. Beta particles are high speed electrons.
Gamma radiation is similar to x-rays (high energy electromagnetic radiation).

In 1910 Rutherford and his co-workers performed an experiment that led to the downfall of Thomson’s model. By 1911 Rutherford postulated that:

• most of the mass of the atom, and all of its positive charge, reside in a very small, extremely dense region which he called the nucleus.
• Most of the total volume of the atom is empty space in which electrons move around the nucleus.
• Electrons occupy most of the volume of the atom but account for only a small fraction of its mass.

The heaviest known atom has a mass of 4 x 10 ^-22

1 atomic mass unit (amu) = 1.666054 x 10^-24g

Atoms are extremely small; most have diameters between 100-500 pm. A convenient unit of length used to express atomic dimensions is the angstrom(A) One angstrom equals 1 x 10^-10 m.

All atoms of an element have the same number of protons in the nucleus (atomic number). Atoms of a given element that differ in the number of neutrons are called isotopes.

The mass number is the total number of protons plus neutrons in the atom.
An atom of a specific isotope is called a nuclide.

¹⁴C carbon fourteen nuclide

A molecule is an assembly of two or more atoms tightly bound together.
Common elements that are diatomic molecules:
H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂

Chemical formulas that indicate the actual numbers and types of atoms in a molecule are called molecular formulas.
Chemical formulas that give only the relative number of atoms of each type in molecule are called empirical formulas. The subscripts in an empirical formula are always the smallest whole-number ratios.

The nucleus of an atom is unchanged by chemical processes, but atoms can readily gain or lose electrons.
If electrons are removed or added to a neutral atom a charged particle called an ion is formed.
An ion with a positive charge is called a cation; a negatively charged ion is called and anion.

Because there is no discrete molecule of NaCl, we are able to write only an empirical formula.

Classification of Compounds (nomenclature)
International Union of Pure and Applied Chemistry IUPAC

Inorganic compounds can be placed into five common classes; binary ionic, ternary ionic binary molecular, binary acid, and ternary oxyacid.

Binary Ionic: two elements, a metal and non -metal. NaCl, KI, AlCl₃, CaF₂

Ternary ionic: three elements, at least one metal and a nonmetal. KNO₃, Al(NO₃)₃, MgSO4

Binary molecular: two elements that are both nonmetals. H₂O, NH₃, CO₂Binary Acids: compounds containing hydrogen and one other nonmetal. HCl, HI, H₂S
Ternary oxyacids: compounds containing hydrogen a nonmetal and oxygen. HNO₃, H₂SO₄, H₃PO₄

Nomenclature of Monoatomic Cations
Main-group metals usually form one cation. Cations are named for the parent metal followed by the word ion. Na⁺ ( sodium ion) Al³⁺ ( aluminum ion)
Transition metals often form more than one cation.
Ex. Fe²⁺ and Fe³⁺
+ It is necessary to specify the charge. Iron(II) and iron(III) ion. Cu⁺ is named copper(I) ion and Cu2+ is named copper(II) ion. This is called the Stock system.
Note that Ag⁺, Zn²⁺, and Cd²⁺ are exceptions (have fixed ionic charge) and do not require Roman numerals.
Latin System or Suffix System
This system takes the Latin name of the metal and adds an -ous or ic suffix. The lower of the two ionic charges receives the -ous suffix, and the higher charge recives the -ic sufix.
The Latin name for iron is ferrum. Fe²⁺ is ferrous while Fe³⁺ is ferric.
The Latin name for copper is cuprum. Add ous or ic to the cupr- stem.
Cu⁺ cuprous ion. Cu²⁺ cupric ion.

IUPAC convention for nonmetal ions use using the nonmetal stem + ide suffix.
Anion IUPAC Name Br- bromide ion
Cl^- chloride ion
F- fluoride ion
I^- iodide ion
N^3- nitride ion
O^2- oxide ion
P^3- phosphide ion
Most polyatomic anions end in an -ate or ite suffix. The -ite suffix has one less oxygen than that for the -ate ending.
There are three exceptions to memorize, OH^-, CN^- (ide) and the ammonium cation (NH4+).

Polyatomic ions

Hypo means under and per means over.

ClO^- hypochlorite ion
ClO₂^- chlorite ion
ClO₃^- chlorate ion
ClO₄^- perchlorate ion

Writing Chemical Formulas

A formula unit is the simplest representative particle in an ionic compound. The total positive charge must equal the total negative charge for the unit to be neutral.

NaCl contains one Na⁺ and one Cl^-

CaCl₂ contains one Ca²⁺ and two Cl^-

Binary compounds

Name the most metallic (less electronegative) element first and the more electronegative element second.

The less metallic (more electronegative) element is named by adding an “-ide” suffix to the element’s unambiguous stem.

B bor C carb N nitr O ox H hydr
Si silic P phosph S sulf F fluor
Se selen Cl chlor
Te tellur Br brom
I iod

Compounds composed of nonmetals only

Nearly all binary molecular compounds involve two nonmetals bonded together. Although many nonmetals can exhibit different ionic charges (oxidation numbers), their oxidation numbers properly are not indicated by Roman numerals or suffixes. Instead, elemental proportions are indicated by using a prefix system for both elements.

Greek prefixes are used to indicate the number of each atom

Formula Name

SO₂ sulfur dioxide
SO₃ sulfur trioxide
N₂O₄ dinitrogen tetroxide
Cl₂O₇ dichlorine heptoxide
CS₂ carbon disulfide
As₄O₆ tetraarsenic hexoxide

Compounds formed between metals and nonmetals.

Binary ionic compounds contain metal cations and nonmetal anions. The cation is named first and the anion second according to the rule described previously.

Formula Name
KBr potassium bromide
CaCl₂ calcium chloride
NaH sodium hydride
RbS rubidium sulfide
Al₂Se₃ aluminum selenide
SrO strontium oxide
Metals with multiple ionic charges
(oxidation states)
The preceding method is sufficient for naming binary ionic compounds containing metals that exhibit only one oxidation number other than zero. Most transition elements, and few of the more electronegative representative metals exhibit more than one oxidation number. To distinguish among all possibilities, the oxidation number of the metal is indicated by a Roman numeral in parentheses following its name.

Compounds containing 3 or more atoms

Formula Name
NH₄I ammonium iodide
Ca(CN)₂ calcium cyanide
NaOH sodium hydroxide
NH₄CN ammonium cyanide
Cu(ClO₃)₂ copper(II) chlorate or
cupric chlorate
Fe₂(SO₄)₃ iron(III) sulfate or
ferric sulfate

Aqueous solutions
Binary acids are compounds in which H is bonded to the more electronegative nonmetals. These compounds act as acids when dissolved in water. The compounds are name as typical binary compounds. Their aqueous solutions are named by modifying the characteristic stem of the nonmetal with the prefix “hydro-“ and the suffix “-ic” followed by the word “acid.”

Name of Aqueous
Formula Name of Compound solution
HCl hydrogen chloride hydrochloric acid
HF hydrogen fluoride hydrofluoric acid
H₂S hydrogen sulfide hydrosulfuric acid
HCN hydrogen cyanide hydrocyanic acid

For ternary and higher compounds, the word “hydrogen” is dropped, and the name of the polyatomic ion is used. “-ate” is replaced with “-ic” and “-ite” is replaced with "-ous”

Formula Name of Compound Aqueous solution
HNO₂ hydrogen nitrite nitrous acid
HNO₃ hydrogen nitrate nitric acid
H₂SO₄ hydrogen sulfate sulfuric acid
H₃PO₃ hydrogen phosphite phosphorous acid
H₃PO₄ hydrogen phosphate phosphoric acid